1. Technical Field
The present invention relates to a flexural vibrator element, and a method of manufacturing the flexural vibrator element. Further, the invention relates to a variety of electronic devices using the flexural vibrator element.
2. Related Art
In the past, in order for detecting a physical quantity such as an angular velocity, an angular acceleration, an acceleration, or force, a sensor such as a piezoelectric vibrating gyroscope using a flexural vibrator element has been used widely in a variety of electronic devices such as a digital still camera, a video camera, a navigation system, a vehicle posture detection device, a pointing device, a game controller, a cellular phone, and a head-mount display. As the flexural vibrator element for the piezoelectric gyroscope, there has been known a tuning-fork flexural vibrator element provided with a driving vibrator arm and a detecting vibrator arm disposed in parallel to each other (see, e.g., JP-A-9-14973 (Patent Document 1), and JP-A-2002-340559 (Patent Document 2)).
In general, in the tuning-fork flexural vibrator element, when applying an alternating-current voltage to an excitation electrode of the driving vibrator arm, the both vibrator arms make flexural vibration inside the plane of the vibrator element. In this state, if the flexural vibrator element moves rotationally around the extension direction of the vibrator arm, the Coriolis force is caused, and the both vibrator arms make flexural vibration in the direction perpendicular to the plane of the vibrator element. The voltage caused between the detection electrodes of the detecting vibrator arm by the operation described above is detected as an electrical signal corresponding to the angular acceleration of the rotational movement of the flexural vibrator element.
It has been known that it is possible for the detecting vibrator arm to detect the voltage caused by the rotational movement of the flexural vibrator element with high sensitivity by providing the detection electrodes on the right and left side surfaces instead of the upper and lower surfaces thereof (see, e.g., Patent Document 2 and JP-A-2007-93400 (Patent Document 3)). FIGS. 11A through 11C show the typical example of such a tuning-fork flexural vibrator element in the related art. The tuning-fork flexural vibrator element 1 is provided with the driving vibrator arm 3 and the detecting vibrator arm 4 extending from a base section 2 having a rectangular shape in parallel to each other. The driving vibrator arm 3 has drive electrodes 5a through 5d respectively on upper and lower surfaces and right and left side surfaces, and when applying an alternating-current voltage to generate electrical fields between the drive electrodes adjacent to each other in the directions alternately reversed, the both vibrator arms vibrate flexurally in the directions opposite to each other along the X-axis direction. The detecting vibrator arm 4 has two pairs of detection electrodes 6a, 7a, and 6b, 7b disposed respectively on the left and right side surfaces so as to be separated in the thickness direction and have polarities different from each other. In the state in which the both vibrator arms 3, 4 are vibrating flexurally in the X-axis direction, if the flexural vibrator element 1 makes a rotational movement around the Y-axis direction, the both vibrator arms vibrate flexurally in the directions opposite to each other along the Z-axis direction. Since the electrical field caused inside the detecting vibrator arm 4 by the operation described above can efficiently be detected between the respective pairs of detection electrodes 6a and 6b, 7a and 7b opposed to each other on the right and left side surfaces as a voltage with a large amplitude, high sensitivity can be obtained.
In general, the electrodes of the flexural vibrator element are formed by patterning the electrode films by the wet etching using the photolithography technology. FIGS. 12A through 12D exemplify the process of forming the detection electrodes 6a, 6b, 7a, 7b on the side surfaces of the detecting vibrator arm 4 in the flexural vibrator element 1 shown in FIGS. 11A through 11C. Firstly, photo etching is performed on the wafer made of, for example, quartz crystal to process the outer shape of the flexural vibrator element 1 to thereby form an element segment, then an electrode film 8 is made to adhere to the surface thereof, and then a photoresist film 9 is applied to the surface thereof. A photomask 10 is disposed on the upper surface of the element segment using a contact method of making the photomask 10 have direct contact with the upper surface of the element segment (FIG. 12A).
The photomask 10 is provided with opening sections 11a through 11c corresponding to the areas of the photoresist film 9 desired to be exposed. The area 9a corresponding to a roughly entire upper surface of the photoresist film 9 is irradiated with ultraviolet light vertically from above through the opening section 11a, and the areas 9b, 9c of the left and right side surfaces located around the center in the thickness direction are irradiated from obliquely above through the opening sections 11b, 11c, respectively. Subsequently, the photomask 10 is disposed on the lower surface of the element segment similarly using the contact method, and then the area 9d corresponding to a roughly entire lower surface of the photoresist film 9 and the areas 9b, 9c of the left and right side surfaces are similarly exposed.
Subsequently, the photoresist film 9 is developed to thereby remove the areas thus exposed and form resist patterns 12, and thus the electrode film 8 is exposed (FIG. 12B). The part of the electrode film thus exposed is removed by wet etching to thereby expose the quartz crystal surface (FIG. 12C). Finally, by completely removing the residual resist patterns 12, the detection electrodes 6a, 6b, 7a, and 7b separated in the thickness direction are formed on the side surfaces of the detecting vibrator arm 4 (FIG. 12D).
Further, there has been known a method of manufacturing a piezoelectric vibrator element in which the detection electrodes separated in the thickness direction are formed on the side surface of the detecting vibrator arm by using a wafer made of a piezoelectric material having an etching rate higher in the thickness direction than in the width direction (see, e.g., Japanese Patent No. 4,010,218 (Patent Document 4)). In this method, the detection electrodes divided in the thickness direction are formed on the side surfaces of the detecting vibrator arm by processing the piezoelectric vibrator element in the outer shape using wet etching and then making an electrode film adhere thereon so that a projecting strip extending in the longitudinal direction remains in the center of the side surfaces of a vibrating vibrator arm, then exposing the photoresist film formed thereon vertically from above and below simultaneously and then developing it to thereby pattern the photoresist film so that the electrode film is exposed on the upper and lower surfaces of the vibrating vibrator arm and at the tip of the projecting strip, and then etching the part of the electrode film thus exposed.
Further, there has been known a method of etching a quartz crystal plate on the both sides thereof so as to leave a part thereof in the thickness direction to thereby form grooves along the outer shape of the vibrator, then forming electrode films on the side surfaces of the grooves, then mechanically breaking the part of the grooves left in the etching process to thereby separate discrete vibrators, and at the same time forming electrodes separated in the thickness direction by the projection remaining in the grooves at that occasion (see, e.g., JP-A-8-18371 (Patent Document 5)). Further, there has been known a method of removing the part of the grooves, which is left in the etching process, using re-etching to thereby divide the electrode in the thickness direction in a similar manner (see, e.g., JP-A-8-162874 (Patent Document 6)).
As the flexural vibrator element for the piezoelectric vibrating gyroscope, there have been proposed those having various structures besides the tuning-fork type described above. For example, there has been known a so-called H-type, namely a double tuning-fork type, flexural vibrator element provided with a pair of driving vibrator arms extending from the base section in parallel to each other and a pair of detecting vibrator arms extending from the base section in parallel to each other in the opposite direction to that of the driving vibrator arms (see, e.g., Patent Documents 2, 3, and JP-A-2004-125458 (Patent Document 7)). Further, there has been known a multilegged flexural vibrator element provided with three or more vibrator arms extending from the base section in parallel to each other (see, e.g., JP-A-2006-262289 (Patent Document 8)).
Further, there has been known a flexural vibrator element having grooves formed respectively on the upper and lower surfaces of the detecting vibrator arm along the longitudinal direction thereof, and having detection electrodes disposed respectively on the right and left inner side surfaces of each of the grooves, in addition to the detection electrodes disposed respectively on the right and left side surfaces of the detecting vibrator arm as described above (see, e.g., Patent Document 7). Since the distance between the detection electrodes opposed to each other in the lateral direction of the vibrator element is reduced by disposing the detection electrodes as described above, the voltage caused between the opposed electrodes can more efficiently be detected, and thus the sensitivity is further improved.
In the driving vibrator arm of the flexural vibrator element, by forming grooves respectively on the upper and lower surfaces along the longitudinal direction thereof, and disposing the drive electrodes, which are disposed respectively on the upper and lower sides, on the inner surfaces of the grooves, it is possible to generate the electrical field in parallel to the upper and lower surfaces between the drive electrodes on the right and left side surfaces opposed to each other to thereby improve the electrical field efficiency and thus raise the Q value, and thus suppress the CI value to a low level (see, e.g., JP-A-2009-189039 (Patent Document 9)). According to Patent Document 9, the groove in the longitudinal direction of the driving vibrator arm can be formed on the upper surface or the lower surface thereof as a step having a step section (the inside surface of the upper surface or the lower surface) and an intermediate surface section. The drive electrode on the upper surface side and the lower surface side disposed inside the groove is formed continuously on the step section and the intermediate surface section of the step.
However, as described above in relation to FIGS. 12A through 12D, in the method of the related art of irradiating the photoresist film with the ultraviolet light obliquely to thereby perform pattering in order for dividing the detection electrode on the side surface of the detection arm in the thickness direction, the distance between the opening section of the photomask and the exposed area of the photoresist film becomes long, and therefore the diffraction is caused in the exposure light. Therefore, the resolution is low, and pattern accuracy might be degraded. As a result, in the wet etching of the electrode film, the amount of side-etching is apt to increase, and therefore, it is difficult to accurately pattern the detection electrode, and the fraction defective in manufacturing increases. In particular, since it is difficult to form fine electrode patterns, downsizing of the flexural vibrator element is hindered. Further, since the exposure in the contact method is required to be performed on each of the upper and lower surfaces of the flexural vibrator element, the manufacturing process becomes complicated, and the life of the photomask is also shortened. Further, a special exposure device is necessary for performing the oblique irradiation of the exposure light. As a result of these circumstances, there arises a problem of rise in the manufacturing cost.
Further, the method of the related art described in Patent Document 4 using the wafer made of the piezoelectric material having the etching rate higher in the thickness direction than in the width direction is advantageous because the exposure of the photoresist film can be performed vertically and simultaneously on the upper and lower sides of the vibrator arm instead of the contact method, and therefore, the manufacturing process is simpler, and a common exposure device can be used. However, there is a problem that available piezoelectric material and crystalline orientation are limited.
The method of the related art described in Patent Document 5 requires an additional process and a special jig for breaking the part of the groove left in the etching process. Therefore, there is a problem that the operation becomes complicated, and growth in the manufacturing cost is resulted in. Further, in breaking the part left in the etching process, fragments of the quartz crystal adheres to the quartz crystal vibrator to alter the vibration characteristics, or have contact with other adjacent quartz crystal vibrator or the jig to damage the electrode film, and thus occurrence of defective products or degradation of yield might be incurred. Further, since it is not achievable to control the position at which the quartz crystal is broken, the size of the projection remaining on the side surface of the vibrator arm becomes uneven, and therefore, the balance of the vibrator arm is lost to thereby cause unwanted vibrations, and the vibration characteristics might be degraded.
The method of the related art described in Patent Document 6 also requires an additional process for completely removing the part left in the etching process using the re-etching, and therefore, there is a problem that the process similarly becomes complicated to thereby incur the growth in the manufacturing cost. Further, there is a possibility that hollows caused by the over etching are formed on the side surface of the vibrator arm due to the crystalline anisotropy of the quartz crystal when re-etching the part left in the etching process, or the electrode material caused when removing the resist film and the electrode film formed on the part left in the etching process is reattached to the electrode.